Liquid-crystalline medium

ABSTRACT

The invention relates to liquid-crystalline mixtures comprising at least one compound of the formula I, 
                         
in which
 
X, L 1 , L 2 , L 3 , L 4  and alkenyl have the meanings indicated in claim  1,  
 
in order to increase the pretilt at the polyimide surface,
 
and to
 
liquid-crystalline media comprising one or more compounds of the formula I in electro-optical liquid-crystal displays and in LC lenses.

The present invention relates to a liquid-crystalline medium (LCmedium), to the use thereof for electro-optical purposes, and to LCdisplays containing this medium.

Liquid crystals are used principally as dielectrics in display devices,since the optical properties of such substances can be modified by anapplied voltage. Electro-optical devices based on liquid crystals areextremely well known to the person skilled in the art and can be basedon various effects. Examples of such devices are cells having dynamicscattering, DAP (deformation of aligned phases) cells, guest/host cells,TN cells having a twisted nematic structure, STN (supertwisted nematic)cells, SBE (super-birefringence effect) cells and OMI (optical modeinterference) cells. The commonest display devices are based on theSchadt-Helfrich effect and have a twisted nematic structure. Inaddition, there are also cells which work with an electric fieldparallel to the substrate and liquid-crystal plane, such as, forexample, IPS (in-plane switching) cells. TN, STN, FFS (fringe fieldswitching) and IPS cells, in particular, are currently commerciallyinteresting areas of application for the media according to theinvention.

The liquid-crystal materials must have good chemical and thermalstability and good stability to electric fields and electromagneticradiation. Furthermore, the liquid-crystal materials should have lowviscosity and produce short addressing times, low threshold voltages andhigh contrast in the cells.

They should furthermore have a suitable mesophase, for example a nematicor cholesteric mesophase for the above-mentioned cells, at the usualoperating temperatures, i.e. in the broadest possible range above andbelow room temperature. Since liquid crystals are generally used asmixtures of a plurality of components, it is important that thecomponents are readily miscible with one another. Further properties,such as the electrical conductivity, the dielectric anisotropy and theoptical anisotropy, have to satisfy various requirements depending onthe cell type and area of application. For example, materials for cellshaving a twisted nematic structure should have positive dielectricanisotropy and low electrical conductivity.

For example, for matrix liquid-crystal displays with integratednon-linear elements for switching individual pixels (MLC displays),media having large positive dielectric anisotropy, broad nematic phases,relatively low birefringence, very high specific resistance, good UV andtemperature stability and low vapour pressure are desired.

Matrix liquid-crystal displays of this type are known. Examples ofnon-linear elements which can be used to individually switch theindividual pixels are active elements (i.e. transistors). The term“active matrix” is then used, where a distinction can be made betweentwo types:

-   1. MOS (metal oxide semiconductor) or other diodes on silicon wafers    as substrate.-   2. Thin-film transistors (TFTs) on a glass plate as substrate.

The use of single-crystal silicon as substrate material restricts thedisplay size, since even modular assembly of various part-displaysresults in problems at the joints.

In the case of the more promising type 2, which is preferred, theelectro-optical effect used is usually the TN effect. A distinction ismade between two technologies: TFTs comprising compound semiconductors,such as, for example, CdSe, or TFTs based on polycrystalline oramorphous silicon. Intensive work is being carried out worldwide on thelatter technology.

The TFT matrix is applied to the inside of one glass plate of thedisplay, while the other glass plate carries the transparentcounterelectrode on its inside. Compared with the size of the pixelelectrode, the TFT is very small and has virtually no adverse effect onthe image. This technology can also be extended to fully colour-capabledisplays, in which a mosaic of red, green and blue filters is arrangedin such a way that a filter element is opposite each switchable pixel.

The TFT displays usually operate as TN cells with crossed polarisers intransmission and are backlit.

The term MLC displays here encompasses any matrix display withintegrated non-linear elements, i.e., besides the active matrix, alsodisplays with passive elements, such as varistors or diodes(MIM=metal-insulator-metal).

MLC displays of this type are particularly suitable for TV applications(for example pocket televisions) or for high-information displays forcomputer applications (laptops) and in automobile or aircraftconstruction. Besides problems regarding the angle dependence of thecontrast and the response times, difficulties also arise in MLC displaysdue to insufficiently high specific resistance of the liquid-crystalmixtures [TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E.,SORIMACHI, K., TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay84, September 1984: A 210-288 Matrix LCD Controlled by Double StageDiode Rings, pp. 141 ff., Paris; STROMER, M., Proc. Eurodisplay 84,September 1984: Design of Thin Film Transistors for Matrix Addressing ofTelevision Liquid Crystal Displays, pp. 145 ff., Paris]. With decreasingresistance, the contrast of an MLC display deteriorates, and the problemof after-image elimination may occur. Since the specific resistance ofthe liquid-crystal mixture generally drops over the life of an MLCdisplay owing to interaction with the interior surfaces of the display,a high (initial) resistance is very important in order to obtainacceptable lifetimes. In particular in the case of low-volt mixtures, itwas hitherto impossible to achieve very high specific resistance values.It is furthermore important that the specific resistance exhibits thesmallest possible increase with increasing temperature and after heatingand/or UV exposure. The low-temperature properties of the mixtures fromthe prior art are also particularly disadvantageous. It is demanded thatno crystallisation and/or smectic phases occur, even at lowtemperatures, and the temperature dependence of the viscosity is as lowas possible. The MLC displays from the prior art thus do not satisfytoday's requirements.

Besides liquid-crystal displays which use backlighting, i.e. areoperated transmissively and if desired transflectively, reflectiveliquid-crystal displays are also particularly interesting. Thesereflective liquid-crystal displays use the ambient light for informationdisplay. They thus consume significantly less energy than backlitliquid-crystal displays having a corresponding size and resolution.Since the TN effect is characterised by very good contrast, reflectivedisplays of this type can even be read well in bright ambientconditions. This is already known of simple reflective TN displays, asused, for example, in watches and pocket calculators. However, theprinciple can also be applied to high-quality, higher-resolution activematrix-addressed displays, such as, for example, TFT displays. Here, asalready in the transmissive TFT-TN displays which are generallyconventional, the use of liquid crystals of low birefringence (Δn) isnecessary in order to achieve low optical retardation (d·Δn). This lowoptical retardation results in usually acceptably low viewing-angledependence of the contrast (cf. DE 30 22 818). In reflective displays,the use of liquid crystals of low birefringence is even more importantthan in transmissive displays since the effective layer thicknessthrough which the light passes is approximately twice as large inreflective displays as in transmissive displays having the same layerthickness.

For TV and video applications, displays having fast response times arerequired in order to be able to reproduce multimedia content, such as,for example, films and video games, in near-realistic quality. Suchshort response times can be achieved, in particular, if liquid-crystalmedia having low values for the viscosity, in particular the rotationalviscosity γ₁, and having high optical anisotropy (Δn) are used.

In the case of TN (Schadt-Helfrich) cells, media are desired whichfacilitate the following advantages in the cells:

-   -   extended nematic phase range (in particular down to low        temperatures)    -   the ability to switch at extremely low temperatures (outdoor        use, automobiles, avionics)    -   increased resistance to UV radiation (longer lifetime)    -   low threshold voltage.

The media available from the prior art do not enable these advantages tobe achieved while simultaneously retaining the other parameters.

In the case of supertwisted (STN) cells, media are desired whichfacilitate greater multiplexability and/or lower threshold voltagesand/or broader nematic phase ranges (in particular at low temperatures).To this end, a further widening of the available parameter latitude(clearing point, smectic-nematic transition or melting point, viscosity,dielectric parameters, elastic parameters) is urgently desired.

In particular in the case of LC displays for TV and video applications(for example LCD TVs, monitors, PDAs, notebooks, games consoles), asignificant reduction in the response times is desired. A reduction inthe layer thickness d (“cell gap”) of the LC medium in the LC celltheoretically results in faster response times, but requires LC mediahaving higher birefringence Δn in order to ensure an adequate opticalretardation (d·Δn). However, the LC materials of high birefringenceknown from the prior art generally also have high rotational viscosityat the same time, which in turn has an adverse effect on the responsetimes.

Highly wavy surface structures in the display or certainpolyimide/polyimide process conditions (rubbing length) may, owing to aninadequate pretilt at the polyimide surface, result in display defects,such as, for example, reverse twist or mura (uneven grey-shadedistribution). It is necessary here systematically to increase thepretilt of the mixture by choosing a different material, but withoutsignificantly impairing other parameters. This often requires a switchof the mixture concept, since the liquid-crystalline substancesavailable often only influence the pretilt slightly.

The object of the present invention is therefore to findliquid-crystalline mixtures which comprise substances which increase thepretilt disproportionately. Furthermore, the mixtures should have fastresponse times, low rotational viscosities and very high birefringence.

The invention is based on the object of providing media, in particularfor MLC, TN, STN, OCB, HT-VA, FFS or IPS displays of this type, whichhave the desired properties indicated above and do not exhibit thedisadvantages mentioned above or only do so to a lesser extent. Inparticular, the LC media should have fast response times and lowrotational viscosities at the same time as high birefringence. Inaddition, the LC media should have a high clearing point, highdielectric anisotropy and in particular high birefringence.

It has now been found that this object can be achieved if LC mediacomprising one or more compounds of the formula I are used. Thecompounds of the formula I result in mixtures having the desiredproperties indicated above and increase the pretilt at the polyimidesurface disproportionately.

The invention thus relates to a liquid-crystalline medium, characterisedin that it comprises one or more compounds of the formula I

in whichX denotes F, Cl, CN, OCN, SF₅, a fluorinated alkyl radical having 1-6 Catoms, a fluorinated alkoxy radical having 1-6 C atoms, a fluorinatedalkenyl radical having 2-6 C atoms, a fluorinated alkylalkoxy radicalhaving 1-6 C atoms,L¹⁻⁴ each, independently of one another, denote H or F, andalkenyl denotes an alkenyl radical having 2-6 C atoms.

Surprisingly, it has been found that LC media comprising compounds ofthe formula I have a very good ratio of rotational viscosity γ₁ andclearing point, a high value for the optical anisotropy Δ∈ and very highbirefringence Δn, as well as fast response times, a low thresholdvoltage, a high clearing point, high positive dielectric anisotropy anda broad nematic phase range. Furthermore, the compounds of the formula Iare very readily soluble in liquid-crystalline media.

The present invention likewise relates to the compounds of the formulaI.

The compounds of the formula I have a broad range of applications.Depending on the choice of substituents, they can serve as basematerials of which liquid-crystalline media are predominantly composed;however, liquid-crystalline base materials from other classes ofcompound can also be added to the compounds of the formula I in order,for example, to modify the dielectric and/or optical anisotropy of adielectric of this type and/or to optimise its threshold voltage and/orits viscosity.

Preferred compounds of the formula I are, in particular, those in whichX denotes fluorine, furthermore OCF₃.

In the compounds of the formula I, “alkenyl” preferably denotes astraight-chain alkenyl radical having 2-5 C atoms, in particularCH₃CH═CHC₂H₄, furthermore H₂C═CHC₂H₄, CH₃CH═CH, H₂C═CH.

L¹⁻⁴ preferably have the meanings L¹=L²=L³=L⁴=F, furthermore L¹=L²=L³=Fand L⁴=H, or L¹=L³=F and L²=L⁴=H.

Preferred compounds of the formula I are indicated below:

Of the said compounds, particular preference is given to the compoundsof the formulae I2 and I5, very particularly those where X═F.

Particularly preferred compounds are indicated below:

Particular preference is given to the compounds of the formulae I2-1 andI5-1.

In the pure state, the compounds of the formula I are colourless andform liquid-crystalline mesophases in a temperature range which isfavourably located for electro-optical use. They are stable chemically,thermally and to light.

The compounds of the formula I are prepared by methods known per se, asdescribed in the literature (for example in the standard works, such asHouben-Weyl, Methoden der organischen Chemie [Methods of OrganicChemistry], Georg-Thieme-Verlag, Stuttgart), to be precise underreaction conditions which are known and suitable for the said reactions.Use can also be made here of variants known per se which are notmentioned here in greater detail.

Preferably the compounds of the formula I are prepared as follows:

Further preferred embodiments of the mixtures according to the inventionare indicated below:

-   -   The medium additionally comprises one or more neutral compounds        of the formulae II and/or III:

-   -   in which    -   A denotes 1,4-phenylene or trans-1,4-cyclohexylene,    -   a denotes 0 or 1,    -   R³ denotes alkenyl having 2 to 9 C atoms, and    -   R⁴ denotes a straight-chain alkyl radical having 1 to 12 C atoms        or alkenyl radical having 2 to 9 C atoms,    -   The compounds of the formula II are preferably selected from the        following formulae:

-   -   in which R^(3a) and R^(4a) each, independently of one another,        denote H, CH₃, C₂H₅ or C₃H₇, and “alkyl” denotes a        straight-chain alkyl group having 1 to 8 C atoms. Particular        preference is given to compounds of the formulae IIa and IIf, in        particular in which R^(3a) denotes H or CH₃, and compounds of        the formula IIc, in particular in which R^(3a) and R^(4a) denote        H, CH₃ or C₂H₅.    -   Preference is furthermore given to compounds of the formula II        which have a non-terminal double bond in the alkenyl side chain:

-   -   Very particularly preferred compounds of the formula II are the        compounds of the formulae

-   -   Besides one or more compounds of the formula I, the        liquid-crystalline media according to the invention particularly        preferably comprise 5-70% by weight of compounds of the formula

-   -   The compounds of the formula III are preferably selected from        the following formulae:

-   -   in which “alkyl” and R^(3a) have the meanings indicated above,        and R^(3a) preferably denotes H or CH₃. Particular preference is        given to compounds of the formula IIIb;    -   The medium preferably additionally comprises one or more        compounds selected from the following formulae IV to VIII:

-   -   in which    -   R⁰ has the meanings indicated in formula I, and    -   X⁰ denotes F, Cl, a mono- or polyfluorinated alkyl or alkoxy        radical, in each case having 1 to 6 C atoms, a mono- or        polyfluorinated alkenyl or alkenyloxy radical, in each case        having 2 to 6 C atoms.    -   Y¹⁻⁶ each, independently of one another, denote H or F,    -   Z⁰ denotes —C₂H₄—, —(CH₂)₄—, —CH═CH—, —CF═CF—, —C₂F₄—, —CH₂CF₂—,        —CF₂CH₂—, —CH₂O—, —OCH₂—, —COO—, —CF₂O— or —OCF₂—, in the        formulae V and VI also a single bond, and    -   r denotes 0 or 1.    -   In the above formulae, X⁰ is preferably F, Cl or a mono- or        polyfluorinated alkyl or alkoxy radical having 1, 2 or 3 C atoms        or a mono- or polyfluorinated alkenyl radical or alkenyloxy        radical having 2 or 3 C atoms. X⁰ is particularly preferably F,        Cl, CF₃, CHF₂, OCF₃, OCHF₂, OCFHCF₃, OCFHCHF₂, OCFHCHF₂,        OCF₂CH₃, OCF₂CHF₂, OCF₂CHF₂, OCF₂CF₂CHF₂, OCF₂CF₂CH₂F,        OCFHCF₂CF₃, OCFHCF₂CHF₂, OCH═CF₂, OCF═CF₂, OCF₂CHFCF₃,        OCF₂CF₂CF₃, OCF₂CF₂CClF₂, OCClFCF₂CF₃, CF═CF₂, CF═CHF, OCH═CF₂,        OCF═CF₂, or CH═CF₂. X⁰ very particularly preferably denotes F or        OCF₃.    -   In the compounds of the formulae IV to VIII, X⁰ preferably        denotes F or OCF₃, furthermore OCHF₂, CF₃, CF₂H, Cl, OCH═CF₂. R⁰        is preferably straight-chain alkyl or alkenyl having up to 6 C        atoms.    -   The compounds of the formula IV are preferably selected from the        following formulae:

-   -   in which R⁰ and X⁰ have the meanings indicated above.    -   Preferably, R⁰ in formula IV denotes alkyl having 1 to 8 C atoms        and X⁰ denotes F, Cl, OCHF₂ or OCF₃, furthermore OCH═CF₂. In the        compound of the formula IVb, R⁰ preferably denotes alkyl or        alkenyl. In the compound of the formula IVd, X⁰ preferably        denotes Cl, furthermore F.    -   The compounds of the formula V are preferably selected from the        following formulae Va to Vj:

-   -   in which R⁰ and X⁰ have the meanings indicated above.        Preferably, R⁰ in formula V denotes alkyl having 1 to 8 C atoms        and X⁰ denotes F;    -   The medium comprises one or more compounds of the formula VI-1

-   -   particularly preferably those selected from the following        formulae:

-   -   in which R⁰ and X⁰ have the meanings indicated above.        Preferably, R⁰ in formula VI denotes alkyl having 1 to 8 C atoms        and X⁰ denotes F, furthermore OCF₃ and CF₃.    -   The medium comprises one or more compounds of the formula VI-2

-   -   particularly preferably those selected from the following        formulae:

-   -   in which R⁰ and X⁰ have the meanings indicated above.    -   Preferably, R⁰ in formula VI denotes alkyl having 1 to 8 C atoms        and X⁰ denotes F;    -   The medium preferably comprises one or more compounds of the        formula VII in which Z⁰ denotes —CF₂O—, —CH₂CH₂— or —COO—,        particularly preferably those selected from the following        formulae:

-   -   in which R⁰ and X⁰ have the meanings indicated above.        Preferably, R⁰ in formula VII denotes alkyl having 1 to 8 C        atoms and X⁰ denotes F, furthermore OCF₃.    -   The compounds of the formula VIII are preferably selected from        the following formulae:

-   -   in which R⁰ and X⁰ have the meanings indicated above. R⁰ in        formula VIII preferably denotes a straight-chain alkyl radical        having 1 to 8 C atoms. X⁰ preferably denotes F.    -   The medium additionally comprises one or more compounds of the        following formula:

-   -   in which R⁰, X⁰, Y¹ and Y² have the meaning indicated above, and

each, independently of one another, denote

-   -   where rings A and B do not both simultaneously denote        cyclohexylene;    -   The compounds of the formula IX are preferably selected from the        following formulae:

-   -   in which R⁰ and X⁰ have the meanings indicated above.        Preferably, R⁰ in formula IX denotes alkyl having 1 to 8 C atoms        and X⁰ denotes F. Particular preference is given to compounds of        the formula IXa;    -   The medium additionally comprises one or more compounds selected        from the following formulae:

-   -   in which R⁰, X⁰ and Y¹⁻⁴ have the meaning indicated in formula        I, and

each, independently of one another, denote

-   -   The compounds of the formulae X and XI are preferably selected        from the following formulae:

-   -   in which R⁰ and X⁰ have the meanings indicated above.        Preferably, R⁰ denotes alkyl having 1 to 8 C atoms and X⁰        denotes F. Particularly preferred compounds are those in which        Y¹ denotes F and Y² denotes H or F, preferably F.    -   The medium additionally comprises one or more compounds of the        following formula:

-   -   in which R¹ and R² each, independently of one another, denote        n-alkyl, alkoxy, oxaalkyl, fluoroalkyl, alkenyloxy or alkenyl,        each having up to 9 C atoms, and preferably each, independently        of one another, denote alkyl having 1 to 8 C atoms. Y¹ denotes H        or F.    -   Preferred compounds of the formula XII are the compounds of the        formulae

-   -   in which    -   alkyl and alkyl* each, independently of one another, denote a        straight-chain alkyl radical having 1 to 6 C atoms, and    -   alkenyl and    -   alkenyl* each, independently of one another, denote a        straight-chain alkenyl radical having 2 to 6 C atoms.    -   Particular preference is given to the compounds of the formulae        XII-1 and XII-3.    -   A particularly preferred compound of the formula XII-3 is the        compound of the formula XII-3a:

-   -   The compounds of the formula XII are preferably employed in        amounts of 3-30% by weight.    -   The medium additionally comprises one or more compounds selected        from the following formulae:

-   -   in which R⁰, X⁰, Y¹ and Y² have the meanings indicated above.        Preferably, R⁰ denotes alkyl having 1 to 8 C atoms and X⁰        denotes F or Cl;    -   The compounds of the formulae XIII and XV are preferably        selected from compounds of the formulae

-   -   in which R⁰ and X⁰ have the meanings indicated above. R⁰        preferably denotes alkyl having 1 to 8 C atoms. In the compounds        of the formula XIII, X⁰ preferably denotes F or Cl.    -   The medium additionally comprises one or more compounds of the        formulae D1, D2 and/or D3:

-   -   in which Y¹, Y², R⁰ and X⁰ have the meaning indicated above.        Preferably, R⁰ denotes alkyl having 1 to 8 C atoms and X⁰        denotes F.    -   Particular preference is given to compounds of the formulae

-   -   in which R⁰ has the meanings indicated above and preferably        denotes straight-chain alkyl having 1 to 6 C atoms, in        particular C₂H₅, n-C₃H₇ or n-C₅H₁₁.    -   The medium additionally comprises one or more compounds of the        following formula:

-   -   in which Y¹, R¹ and R² have the meanings indicated above. R¹ and        R² preferably each, independently of one another, denote alkyl        having 1 to 8 C atoms.    -   The medium additionally comprises one or more compounds of the        following formula:

-   -   in which X⁰, Y¹ and Y² have the meanings indicated above, and        “alkenyl” denotes C₂₋₇-alkenyl. Particular preference is given        to compounds of the following formula:

-   -   in which R^(3a) has the meaning indicated above and preferably        denotes H;    -   The medium additionally comprises one or more tetracyclic        compounds selected from the formulae XIX to XXIX:

-   -   in which Y¹⁻⁴, R⁰ and X⁰ each, independently of one another,        have one of the meanings indicated above. X⁰ is preferably F,        Cl, CF₃, OCF₃ or OCHF₂. R⁰ preferably denotes alkyl, alkoxy,        oxaalkyl, fluoroalkyl or alkenyl, each having up to 8 C atoms.    -   In the formulae indicated above and below,

preferably denotes

-   -   R⁰ and R⁰* in the formulae indicated above and below are        preferably straight-chain alkyl or alkenyl having 2 to 7 C        atoms;    -   X⁰ in the formulae indicated above and below is preferably F,        furthermore OCF₃, Cl or CF₃;    -   The medium preferably comprises one, two or three compounds of        the formula I, in particular at least one compound of the        formula I2 and/or I5;    -   The medium preferably comprises one or more compounds selected        from the group of the compounds of the formulae I, II, III, V,        VI-1, VI-2, XII, XIII, XIV, XVII, XXIII, XXV;    -   The medium preferably comprises one or more compounds of the        formula VI-1;    -   The medium preferably comprises one or more compounds of the        formula VI-2;    -   The medium preferably comprises 0.5-10% by weight, preferably        1-5% by weight, particularly preferably 1-3% by weight, of        compounds of the formula I;    -   The proportion of compounds of the formulae II-XXVII in the        mixture as a whole is preferably 20 to 99.5% by weight;    -   The medium preferably comprises 25-80% by weight, particularly        preferably 30-70% by weight, of compounds of the formulae II        and/or III;    -   The medium preferably comprises 5-40% by weight, particularly        preferably 10-30% by weight, of compounds of the formula V;    -   The medium preferably comprises 3-30% by weight, particularly        preferably 6-25% by weight, of compounds of the formula VI-1;    -   The medium preferably comprises 2-30% by weight, particularly        preferably 4-25% by weight, of compounds of the formula VI-2;    -   The medium comprises 5-40% by weight, particularly preferably        10-30% by weight, of compounds of the formula XII;    -   The medium preferably comprises 1-25% by weight, particularly        preferably 2-15% by weight, of compounds of the formula XIII;    -   The medium preferably comprises 5-45% by weight, particularly        preferably 10-35% by weight, of compounds of the formula XIV;    -   The medium preferably comprises 1-20% by weight, particularly        preferably 2-15% by weight, of compounds of the formula XVI;    -   The medium additionally comprises one or more compounds of the        formulae St-1 to St-3:

-   -   in which R⁰, Y¹, Y² and X⁰ have the meanings indicated above. R⁰        preferably denotes straight-chain alkyl, preferably having 1-6 C        atoms. X⁰ is preferably F or OCF₃. Y¹ preferably denotes F. Y²        preferably denotes F. Preference is furthermore given to        compounds in which Y¹═F and Y²═H. The compounds of the formulae        St-1 to St-3 are preferably employed in the mixtures according        to the invention in concentrations of 3-30% by weight, in        particular 5-25% by weight.    -   The medium additionally comprises one or more pyrimidine or        pyridine compounds of the formulae Py-1 to Py-5:

-   -   in which R⁰ preferably denotes straight-chain alkyl having 2-5 C        atoms. x denotes 0 or 1, preferably x=1. Preferred mixtures        comprise 3-30% by weight, in particular 5-20% by weight, of        these pyri(mi)dine compound(s).

It has been found that even a relatively small proportion of compoundsof the formula I mixed with conventional liquid-crystal materials, butin particular with one or more compounds of the formulae II to XXVII,results in a significant increase in the light stability and in lowbirefringence values, with broad nematic phases with low smectic-nematictransition temperatures being observed at the same time, improving theshelf life. At the same time, the mixtures exhibit very low thresholdvoltages and very good values for the VHR on exposure to UV.

The term “alkyl” or “alkyl*” in this application encompassesstraight-chain and branched alkyl groups having 1-7 carbon atoms, inparticular the straight-chain groups methyl, ethyl, propyl, butyl,pentyl, hexyl and heptyl. Groups having 1-6 carbon atoms are generallypreferred.

The term “alkenyl” or “alkenyl*” in this application encompassesstraight-chain and branched alkenyl groups having 2-7 carbon atoms, inparticular the straight-chain groups. Preferred alkenyl groups areC₂-C₇-1E-alkenyl, C₄-C₇-3E-alkenyl, C₅-C₇-4-alkenyl, C₆-C₇-5-alkenyl andC₇-6-alkenyl, in particular C₂-C₇-1E-alkenyl, C₄-C₇-3E-alkenyl andC₅-C₇-4-alkenyl. Examples of particularly preferred alkenyl groups arevinyl, 1E-propenyl, 1E-butenyl, 1E-pentenyl, 1E-hexenyl, 1E-heptenyl,3-butenyl, 3E-pentenyl, 3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl,4E-hexenyl, 4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groupshaving up to 5 carbon atoms are generally preferred.

The term “fluoroalkyl” in this application encompasses straight-chaingroups having at least one fluorine atom, preferably a terminalfluorine, i.e. fluoromethyl, 2-fluoroethyl, 3-fluoropropyl,4-fluorobutyl, 5-fluoropentyl, 6-fluorohexyl and 7-fluoroheptyl.However, other positions of the fluorine are not excluded.

The term “oxaalkyl” or “alkoxy” in this application encompassesstraight-chain radicals of the formula C_(n)H_(2n+1)—O—(CH₂)_(m), inwhich n and m each, independently of one another, denote 1 to 6. m mayalso denote 0. Preferably, n=1 and m=1-6 or m=0 and n=1-3.

Through a suitable choice of the meanings of R⁰ and X⁰, the addressingtimes, the threshold voltage, the steepness of the transmissioncharacteristic lines, etc., can be modified in the desired manner. Forexample, 1E-alkenyl radicals, 3E-alkenyl radicals, 2E-alkenyloxyradicals and the like generally result in shorter addressing times,improved nematic tendencies and a higher ratio between the elasticconstants k₃₃ (bend) and k₁₁ (splay) compared with alkyl and alkoxyradicals. 4-Alkenyl radicals, 3-alkenyl radicals and the like generallygive lower threshold voltages and lower values of k₃₃/k₁₁ compared withalkyl and alkoxy radicals. The mixtures according to the invention aredistinguished, in particular, by high K₁ values and thus havesignificantly faster response times than the mixtures from the priorart.

The optimum mixing ratio of the compounds of the above-mentionedformulae depends substantially on the desired properties, on the choiceof the components of the above-mentioned formulae and on the choice ofany further components that may be present.

Suitable mixing ratios within the range indicated above can easily bedetermined from case to case.

The total amount of compounds of the above-mentioned formulae in themixtures according to the invention is not crucial. The mixtures cantherefore comprise one or more further components for the purposes ofoptimisation of various properties. However, the observed effect on thedesired improvement in the properties of the mixture is generallygreater, the higher the total concentration of compounds of theabove-mentioned formulae.

In a particularly preferred embodiment, the media according to theinvention comprise compounds of the formulae IV to VIII in which X⁰denotes F, OF₃, OCF₃, OCHF₂, OCH═CF₂, OCF═CF₂ or OCF₂—CF₂H. A favourablesynergistic action with the compounds of the formula I results inparticularly advantageous properties. In particular, mixtures comprisingcompounds of the formulae I and VI, or I and XI, or I and VI and XI aredistinguished by their low threshold voltages.

The individual compounds of the above-mentioned formulae and thesub-formulae thereof which can be used in the media according to theinvention are either known or can be prepared analogously to the knowncompounds.

The invention also relates to electro-optical displays, such as, forexample, TN, STN, TFT, OCB, IPS, FFS, HT-VA or MLC displays, having twoplane-parallel outer plates, which, together with a frame, form a cell,integrated non-linear elements for switching individual pixels on theouter plates, and a nematic liquid-crystal mixture having positivedielectric anisotropy and high specific resistance located in the cell,which contain media of this type, and to the use of these media forelectro-optical purposes.

Furthermore, the mixtures according to the invention are also suitablefor positive VA applications, also referred to as HT-VA applications.These are taken to mean electro-optical displays having an in-planedrive electrode configuration and homeotropic arrangement of theliquid-crystal medium having positive anisotropy.

The mixtures according to the invention are particularly suitable forTN-TFT display applications having a low operation voltage, i.e.particularly preferably for notebook applications.

The liquid-crystal mixtures according to the invention enable asignificant broadening of the available parameter latitude. Theachievable combinations of clearing point, viscosity at low temperature,thermal and UV stability and high optical anisotropy are far superior toprevious materials from the prior art.

The mixtures according to the invention are particularly suitable formobile applications and high-Δn TFT applications, such as, for example,PDAs, notebooks, LCD TVs and monitors, and also owing to their highbirefringence for 3D lens applications.

The liquid-crystal mixtures according to the invention, while retainingthe nematic phase down to −20° C. and preferably down to −30° C.,particularly preferably down to −40° C., and the clearing point≧70° C.,preferably ≧75° C., at the same time allow rotational viscosities γ₁ of≦120 mPa·s, particularly preferably 100 mPa·s, to be achieved, enablingexcellent MLC displays having fast response times to be achieved.

The dielectric anisotropy Δ∈ of the liquid-crystal mixtures according tothe invention is preferably ≧+8, particularly preferably ≧+12. Inaddition, the mixtures are characterised by low operating voltages. Thethreshold voltage of the liquid-crystal mixtures according to theinvention is preferably ≦1.5 V, in particular ≦1.2 V.

The birefringence Δn of the liquid-crystal mixtures according to theinvention is preferably ≧0.08, in particular ≧0.10 and particularlypreferably ≧0.11.

The nematic phase range of the liquid-crystal mixtures according to theinvention preferably has a width of at least 90°, in particular at least100°. This range preferably extends at least from −25° C. to +70° C.

If the mixtures according to the invention are used in FFS applications,the mixtures preferably have a value of the dielectric anisotropy of3-12 and a value of the optical anisotropy of 0.07-0.13.

It goes without saying that, through a suitable choice of the componentsof the mixtures according to the invention, it is also possible forhigher clearing points (for example above 100° C.) to be achieved athigher threshold voltages or lower clearing points to be achieved atlower threshold voltages with retention of the other advantageousproperties. At viscosities correspondingly increased only slightly, itis likewise possible to obtain mixtures having a higher Δ∈ and thus lowthresholds. The MLC displays according to the invention preferablyoperate at the first Gooch and Tarry transmission minimum [C. H. Goochand H. A. Tarry, Electron. Lett. 10, 2-4, 1974; C. H. Gooch and H. A.Tarry, Appl. Phys., Vol. 8, 1575-1584, 1975], where, besidesparticularly favourable electro-optical properties, such as, forexample, high steepness of the characteristic line and low angledependence of the contrast (German patent 30 22 818), lower dielectricanisotropy is sufficient at the same threshold voltage as in ananalogous display at the second minimum. This enables significantlyhigher specific resistance values to be achieved using the mixturesaccording to the invention at the first minimum than in the case ofmixtures comprising cyano compounds. Through a suitable choice of theindividual components and their proportions by weight, the personskilled in the art is able to set the birefringence necessary for apre-specified layer thickness of the MLC display using simple routinemethods.

The structure of the MLC display according to the invention frompolarisers, electrode base plates and surface-treated electrodescorresponds to the usual design for displays of this type. The termusual design is broadly drawn here and also encompasses all derivativesand modifications of the MLC display, in particular including matrixdisplay elements based on poly-Si TFTs or MIM.

A significant difference between the displays according to the inventionand the hitherto conventional displays based on the twisted nematic cellconsists, however, in the choice of the liquid-crystal parameters of theliquid-crystal layer.

The liquid-crystal mixtures which can be used in accordance with theinvention are prepared in a manner conventional per se, for example bymixing one or more compounds of the formula I with one or more compoundsof the formulae II-XXVII or with further liquid-crystalline compoundsand/or additives. In general, the desired amount of the components usedin the smaller amount is dissolved in the components making up theprincipal constituent, advantageously at elevated temperature. It isalso possible to mix solutions of the components in an organic solvent,for example in acetone, chloroform or methanol, and to remove thesolvent again, for example by distillation, after thorough mixing.

The dielectrics may also comprise further additives known to the personskilled in the art and described in the literature, such as, forexample, UV stabilisers, such as Tinuvin® from Ciba Chemicals,antioxidants, free-radical scavengers, nanoparticles, etc. For example,0-15% of pleochroic dyes or chiral dopants can be added. Suitablestabilisers and dopants are mentioned below in Tables C and D.

In order to set the desired tilt angle locally, polymerisable compoundsmay also additionally be added to the mixtures according to theinvention. Preferred polymerisable compounds are listed in Table E.

In the present application and in the examples below, the structures ofthe liquid-crystal compounds are indicated by means of acronyms, thetrans-formation into chemical formulae taking place in accordance withTable A. All radicals C_(n)H_(2n+1) and C_(m)H_(2m+1) are straight-chainalkyl radicals having n and m C atoms respectively; n, m and k areintegers and preferably denote 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or12. The coding in Table B is self-evident. In Table A, only the acronymfor the parent structure is indicated. In individual cases, the acronymfor the parent structure is followed, separated by a dash, by a code forthe substituents R¹*, R²*, L¹* and L²*:

Code for R¹*, R²*, L¹*, L²*, L³* R¹* R²* L¹* L²* nm C_(n)H_(2n+1)C_(m)H_(2m+1) H H nOm C_(n)H_(2n+1) OC_(m)H_(2m+1) H H nO.mOC_(n)H_(2n+1) C_(m)H_(2m+1) H H n C_(n)H_(2n+1) CN H H nN.FC_(n)H_(2n+1) CN F H nN.F.F C_(n)H_(2n+1) CN F F nF C_(n)H_(2n+1) F H HnCl C_(n)H_(2n+1) Cl H H nOF OC_(n)H_(2n+1) F H H nF.F C_(n)H_(2n+1) F FH nF.F.F C_(n)H_(2n+1) F F F nOCF₃ C_(n)H_(2n+1) OCF₃ H H nOCF₃.FC_(n)H_(2n+1) OCF₃ F H n-Vm C_(n)H_(2n+1) —CH═CH—C_(m)H_(2m+1) H H nV-VmC_(n)H_(2n+1)—CH═CH— —CH═CH—C_(m)H_(2m+1) H H

Preferred mixture components are shown in Tables A and B.

TABLE A

TABLE B

Particular preference is given to liquid-crystalline mixtures which,besides the compounds of the formula I, comprise at least one, two,three, four or more compounds from Table B.

TABLE C Table C indicates possible dopants which are generally added tothe mixtures according to the invention. The mixtures preferablycomprise 0-10% by weight, in particular 0.01-5% by weight andparticularly preferably 0.01-3% by weight of dopants.

TABLE D Stabilisers which can be added, for example, to the mixturesaccording to the invention in amounts of 0-10% by weight are mentionedbelow.

TABLE E Polymerisable compounds which can be added, for example, to themixtures according to the invention in amounts of 0.5-5% by weight aregiven below. If necessary, an initiator must also be added for thepolymerisation in amounts of 0-1%.

RM-1

RM-2

RM-3

RM-4

RM-5

RM-6

RM-7

RM-8

RM-9

RM-10

RM-11

RM-12

RM-13

RM-14

RM-15

RM-16

RM-17

RM-18

RM-19

RM-20

RM-21

RM-22

RM-23

RM-24

RM-25

RM-26

RM-27

RM-28

RM-29

RM-30

RM-31

RM-32

RM-33

The following examples are intended to explain the invention withoutrestricting it.

EXAMPLES

“Conventional work-up” means: water is added if necessary, the mixtureis extracted with methylene chloride, diethyl ether, methyl tert-butylether or toluene, the phases are separated, the organic phase is driedand evaporated, and the product is purified by distillation underreduced pressure or crystallisation and/or chromatography. The followingabbreviations are used:

DAST diethylaminosulfur trifluoride

DCC dicyclohexylcarbodiimide

DDQ dichlorodicyanobenzoquinone

DIBALH diisobutylaluminium hydride

KOT potassium tertiary-butoxide

RT room temperature

THF tetrahydrofuran

pTSOH p-toluenesulfonic acid

TMEDA tetramethylethylenediamine

EXAMPLE 1

Step 1.1

0.13 mol of isopropylmagnesium chloride (2 mol in THF) is added to 0.01mol of magnesium in 700 ml of dry THF at 20° C. The mixture is stirredfor a further 20 min, and a solution of 0.11 mol of A in 200 ml of THFis then added dropwise at 20° C.

After 1 h at room temperature, a solution of 0.13 mol offormylpiperidine in 100 ml of THF is then added dropwise at the sametemperature. The mixture is stirred at room temperature for a further 1h and then hydrolysed using 300 ml of 1 N HCl. The product is isolatedby extraction in a conventional manner and recrystallised from n-heptaneat −20° C.

Step 1.2

0.053 mol of ethyltriphenylphosphonium bromide is suspended in 80 ml ofTHF, and 0.05 mol of potassium tertiary-butoxide, dissolved in 50 ml ofTHF, is added dropwise at 0° C. After 30 min at 0° C., 0.045 mol of B,dissolved in THF, is added dropwise at this temperature.

After hydrolysis using water, the mixture is worked up by extraction ina conventional manner, and the product (Z/E mixture) is isolated bychromatography on silica gel using hexane.

This Z/E mixture is dissolved in 40 ml of butyronitrile, 14 ml of 1 MHCl are added, and 3 portions of benzenesulfinic acid sodium salt (0.005mol each) are added at intervals of 4 h at 60° C. The mixture is stirredfor a further 8 h and worked up by extraction in a conventional manner,and the product is isolated by chromatography on silica gel usinghexane. The pure E product is obtained by recrystallisation fromethanol.

The following compounds of the formula

are prepared analogously:

L¹ L² L³ L⁴ X H H H H F F H H H F F F H H F F F F H F F H F H F F H F FF H H F F F H H H H OCF₃ F H H H OCF₃ F F H H OCF₃ F F F H OCF₃ F F F FOCF₃ F H F H OCF₃ F H F F OCF₃ H H F F OCF₃ H H H H Cl F H H H Cl F F HH Cl F F F H Cl F F F F Cl F H F H Cl F H F F Cl H H F F Cl H H H H CN FH H H CN F F H H CN F F F H CN F F F F CN F H F H CN F H F F CN H H F FCN H H H H OCHF₂ F H H H OCHF₂ F F H H OCHF₂ F F F H OCHF₂ F F F F OCHF₂F H F H OCHF₂ F H F F OCHF₂ H H F F OCHF₂ H H H H OCH═CF₂ F H H HOCH═CF₂ F F H H OCH═CF₂ F F F H OCH═CF₂ F F F F OCH═CF₂ F H F H OCH═CF₂F H F F OCH═CF₂ H H F F OCH═CF₂ H H H H OCF═CF₂ F H H H CF₃ F F H H CF₃F F F H CF₃ F F F F CF₃ F H F H CF₃ F H F F CF₃ H H F F CF₃ H H H H CF₃

EXAMPLE 2

0.12 mol of magnesium turnings is initially introduced in 10 ml of THF,and a solution of 0.12 mol of E-5-bromopent-2-ene in THF is added at theboiling point. The mixture is heated at the boil for a further 1 h, and0.13 mol of anhydrous zinc chloride, dissolved in THF, is then added.0.08 mol of A, dissolved in THF, is subsequently added dropwise withoutcooling. After addition of 0.0024 mol of Pd-DPPF catalyst, thetemperature rises to about 57° C. The mixture is heated at the boil fora further 3 h, hydrolysed by addition of water and worked up byextraction in a conventional manner. The product is isolated bychromatography on silica gel using pentane, and the pure E material isobtained by preparative HPLC on RP silica gel using anacetonitrile/water mixture.

C 25 I; Δn=0.0960; Δ∈=19.1; γ₁=38

The following compounds of the formula

are prepared analogously:

L¹ L² L³ L⁴ X H H H H F F H H H F F F H H F F F F H F F H F H F F H F FF H H F F F H H H H OCF₃ F H H H OCF₃ F F H H OCF₃ F F F H OCF₃ F F F FOCF₃ F H F H OCF₃ F H F F OCF₃ H H F F OCF₃ H H H H Cl F H H H Cl F F HH Cl F F F H Cl F F F F Cl F H F H Cl F H F F Cl H H F F Cl H H H H CN FH H H CN F F H H CN F F F H CN F F F F CN F H F H CN F H F F CN H H F FCN H H H H OCHF₂ F H H H OCHF₂ F F H H OCHF₂ F F F H OCHF₂ F F F F OCHF₂F H F H OCHF₂ F H F F OCHF₂ H H F F OCHF₂ H H H H OCH═CF₂ F H H HOCH═CF₂ F F H H OCH═CF₂ F F F H OCH═CF₂ F F F F OCH═CF₂ F H F H OCH═CF₂F H F F OCH═CF₂ H H F F OCH═CF₂ F H H H CF₃ F F H H CF₃ F F F H CF₃ F FF F CF₃ F H F H CF₃ F H F F CF₃ H H F F CF₃ H H H H CF₃

The following mixture examples are intended to explain the inventionwithout restricting it.

Above and below, percentage data denote percent by weight. Alltemperatures are indicated in degrees Celsius. m.p. denotes meltingpoint, cl.p.=clearing point. Furthermore, C=crystalline state, N=nematicphase, S=smectic phase and I=isotropic phase. The data between thesesymbols represent the transition temperatures. Furthermore,

-   -   Δn denotes the optical anisotropy at 589 nm and 20° C.,    -   γ₁ denotes the rotational viscosity (mPa·s) at 20° C.,    -   V₁₀ denotes the voltage (V) for 10% transmission (viewing angle        perpendicular to the plate surface), (threshold voltage),    -   Δ∈ denotes the dielectric anisotropy at 20° C. and 1 kHz        (Δ∈=∈_(∥)−∈_(⊥), where ∈_(∥) denotes the dielectric constant        parallel to the longitudinal axes of the molecules and ∈_(⊥)        denotes the dielectric constant perpendicular thereto).

The electro-optical data are measured in a TN cell at the 1st minimum(i.e. at a d·Δn value of 0.5 μm) at 20° C., unless expressly indicatedotherwise. The optical data are measured at 20° C., unless expresslyindicated otherwise. All physical properties are determined inaccordance with “Merck Liquid Crystals, Physical Properties of LiquidCrystals”, status November 1997, Merck KGaA, Germany, and apply for atemperature of 20° C., unless explicitly indicated otherwise.

EXAMPLE M1

PCH-5F 10.00% Clearing point [° C.]: 92.5 PCH-6F 8.00% Δn [589 nm, 20°C.]: 0.0968 PCH-7F 6.00% Δε [1 kHz, 20° C.] +5.3 CCP-2OCF₃ 8.00% V₁₀[V]: 2.01 CCP-3OCF₃ 12.00% CCP-4OCF₃ 7.00% CCP-5OCF₃ 11.00% BCH-3F.F12.00% BCH-5F.F 10.00% ECCP-3OCF₃ 5.00% ECCP-5OCF₃ 5.00% CBC-33F 2.00%CBC-53F 2.00% CBC-55F 2.00%

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows that the alkenyl compounds of the formula I, such as, forexample, UQU-1V-F, disproportionately increase the pretilt in Mixture M1compared with the corresponding alkyl compounds.

EXAMPLE M2

CC-3-V 32.00% Clearing point [° C.]: 80.5 UQU-1V-F 5.00% S → N [° C.]:−40 PGP-2-2V 2.00% Δn [589 nm, 20° C.]: 0.1297 CCP-V-1 10.00% Δε [1 kHz,20° C.]: +16.1 PGU-3-F 7.00% V₁₀ [V]: 1.05 BCH-3F.F.F 10.00% APUQU-3-F9.00% PGUQU-3-F 5.00% PGUQU-4-F 6.00% PGUQU-5-F 7.00% DPGU-4-F 7.00%

EXAMPLE M3

CC-3-V 28.50% Clearing point [° C.]: 82.0 UQU-1V-F 10.00% S → N [° C.]:−30 PGP-2-2V 1.00% Δn [589 nm, 20° C.]: 0.1304 CCP-V-1 12.50% Δε [1 kHz,20° C.]: +15.4 PGU-3-F 6.00% BCH-3F.F.F 8.00% APUQU-3-F 8.00% PGUQU-3-F5.00% PGUQU-4-F 5.00% PGUQU-5-F 6.00% DPGU-4-F 7.00% CPGP-4-3 3.00%

EXAMPLE M4

CC-3-V 30.00% Clearing point [° C.]: 81.0 UQU-1V2-F 5.00% S → N [° C.]:−30 PGP-2-2V 1.00% Δn [589 nm, 20° C.]: 0.1300 CCP-V-1 13.00% Δε [1 kHz,20° C.]: +15.8 PGU-3-F 9.00% V₁₀ [V]: 1.09 BCH-3F.F.F 8.00% APUQU-3-F7.00% PGUQU-3-F 5.00% PGUQU-4-F 6.00% PGUQU-5-F 8.00% DPGU-4-F 8.00%

The invention claimed is:
 1. A liquid-crystalline medium, comprising atleast one compound of formula I,

in which X denotes F, a fluorinated alkyl radical having 1-6 C atoms, afluorinated alkoxy radical having 1-6 C atoms, a fluorinated alkenylradical having 2-6 C atoms, a fluorinated alkylalkoxy radical having 1-6C atoms, L¹⁻⁴ each, independently of one another, denote H or F, andalkenyl denotes an alkenyl radical having 2-6 C atoms, and at least onecompound of formulae II and/or III:

in which A denotes 1,4-phenylene or trans-1,4-cyclohexylene, a denotes1, R³ denotes alkenyl having 2 to 9 C atoms, and R⁴ denotes astraight-chain alkyl radical having 1 to 12 C atoms or alkenyl radicalhaving 2 to 9 C atoms; provided that the one or more of the followingconditions are met: the liquid crystalline medium contains at least onecompound of formula I wherein L¹⁻⁴ have one of the following meaningsL¹=L²=L³=L⁴=F, L¹=L²=L³=L⁴=F and L⁴=H, or L¹=L³=F and L²=L⁴=H, and theliquid crystalline medium contains at least one compound of formula Iwherein alkenyl is a straight-chain alkenyl having 2 to 5 carbon atoms.2. The liquid-crystalline medium according to claim 1, additionallycomprising at least one compound of the formulae

in which R^(3a) and R^(4a) each, independently of one another, denote H,CH₃, C₂H₅ or C₃H₇, and “alkyl” denotes a straight-chain alkyl grouphaving 1 to 8 C atoms.
 3. The liquid-crystalline medium according toclaim 1, additionally comprising at least one compound of the formulaeIV to VIII:

in which R⁰ denotes an alkyl or alkoxy radical having 1 to 15 C atoms,where, in addition, one or more CH₂ groups in these radicals may each bereplaced, independently of one another, by —C≡C—, —CF₂O—, —CH═CH—,

—O—, —CO—O— or —O—CO— in such a way that O atoms are not linked directlyto one another, and in which, in addition, one or more H atoms may bereplaced by halogen, X⁰ denotes F, Cl, a mono- or polyfluorinated alkylor alkoxy radical having 1 to 6 C atoms, a mono- or polyfluorinatedalkenyl or alkenyloxy radical having 2 to 6 C atoms, Y¹⁻⁶ each,independently of one another, denote H or F, Z⁰ denotes —C₂H₄—,—(CH₂)₄—, —CH═CH—, —CF═CF—, —C₂F₄—, —CH₂CF₂—, —CF₂CH₂—, —CH₂O—, —OCH₂—,—COO—, —CF₂O— or —OCF₂—, in the formulae V and VI also a single bond,and r denotes 0 or
 1. 4. The liquid-crystalline medium according toclaim 1, additionally comprising at least one compound of the formulaeVa to Vj:

in which R⁰ and X⁰ have the meanings indicated below, R⁰ denotes analkyl or alkoxy radical having 1 to 15 C atoms, where, in addition, oneor more CH₂ groups in these radicals may each be replaced, independentlyof one another, by —C≡C—, —CF₂O—, —CH═CH—,

—O—, —CO—O— or —O—CO— in such a way that O atoms are not linked directlyto one another, and in which, in addition, one or more H atoms may bereplaced by halogen, X⁰ denotes F, Cl, a mono- or polyfluorinated alkylor alkoxy radical having 1 to 6 C atoms, a mono- or polyfluorinatedalkenyl or alkenyloxy radical having 2 to 6 C atoms.
 5. Theliquid-crystalline medium according to claim 1, additionally comprisingat least one compound of the formulae VI-1a to VI-1d:

in which R⁰ and X⁰ have the meanings indicated below, R⁰ denotes analkyl or alkoxy radical having 1 to 15 C atoms, where, in addition, oneor more CH₂ groups in these radicals may each be replaced, independentlyof one another, by —C≡C—, —CF₂O—, —CH═CH—,

—O—, —CO—O— or —O—CO— in such a way that O atoms are not linked directlyto one another, and in which, in addition, one or more H atoms may bereplaced by halogen, X⁰ denotes F, Cl, a mono- or polyfluorinated alkylor alkoxy radical having 1 to 6 C atoms, a mono- or polyfluorinatedalkenyl or alkenyloxy radical having 2 to 6 C atoms.
 6. Theliquid-crystalline medium according to claim 1, additionally comprisingat least one compound of the formulae VI-2a to VI-2f:

in which R⁰ and X⁰ have the meanings indicated below, R⁰ denotes analkyl or alkoxy radical having 1 to 15 C atoms, where, in addition, oneor more CH₂ groups in these radicals may each be replaced, independentlyof one another, by —C≡C—, —CF₂O—, —CH═CH—,

—O—, —CO—O— or —O—CO— in such a way that O atoms are not linked directlyto one another, and in which, in addition, one or more H atoms may bereplaced by halogen, X⁰ denotes F, Cl, a mono- or polyfluorinated alkylor alkoxy radical having 1 to 6 C atoms, a mono- or polyfluorinatedalkenyl or alkenyloxy radical having 2 to 6 C atoms.
 7. Theliquid-crystalline medium according to claim 1, additionally comprisingat least one compound of the formulae X and/or XI:

in which R⁰ and X⁰ have the meanings indicated below, R⁰ denotes analkyl or alkoxy radical having 1 to 15 C atoms, where, in addition, oneor more CH₂ groups in these radicals may each be replaced, independentlyof one another, by —C≡C—, —CF₂O—, —CH═CH—,

—O—, —CO—O— or —O—CO— in such a way that O atoms are not linked directlyto one another, and in which, in addition, one or more H atoms may bereplaced by halogen, X⁰ denotes F, Cl, a mono- or polyfluorinated alkylor alkoxy radical having 1 to 6 C atoms, a mono- or polyfluorinatedalkenyl or alkenyloxy radical having 2 to 6 C atoms, Y¹⁻⁴ each,independently of one another, denote H or F, and

each, independently of one another, denote


8. The liquid-crystalline medium according to claim 1, additionallycomprising at least one compound of the formula XII,

in which R¹ and R² each, independently of one another, denote n-alkyl,alkoxy, oxaalkyl, fluoroalkyl, alkenyloxy or alkenyl, each having up to9 C atoms, and Y¹ denotes H or F.
 9. The liquid-crystalline mediumaccording to claim 1, additionally comprising at least one compound ofthe following formulae:

in which R⁰, X⁰, Y¹ and Y² have the meanings indicated below, R⁰ denotesan alkyl or alkoxy radical having 1 to 15 C atoms, where, in addition,one or more CH₂ groups in these radicals may each be replaced,independently of one another, by —C≡C—, —CF₂O—, —CH═CH—,

—O—, —CO—O— or —O—CO— in such a way that O atoms are not linked directlyto one another, and in which, in addition, one or more H atoms may bereplaced by halogen, X⁰ denotes F, Cl, a mono- or polyfluorinated alkylor alkoxy radical having 1 to 6 C atoms, a mono- or polyfluorinatedalkenyl or alkenyloxy radical having 2 to 6 C atoms, Y¹ denotes H or FY² denotes H or F.
 10. The liquid-crystalline medium according to claim1, comprising 0.5-10% by weight of compounds of the formula I.
 11. Theliquid-crystalline medium according to claim 1, additionally comprisingone or more UV stabilisers and/or antioxidants.
 12. A liquid-crystallinemedium according to claim 1 within an electro-optical device.
 13. Aliquid-crystalline medium according to claim 12 within an active-matrixdisplay, passive-matrix display or a liquid-crystal lens.
 14. Anelectro-optical liquid-crystal display containing a liquid-crystallinemedium according to claim
 1. 15. A process for the preparation of aliquid-crystalline medium according to claim 1, comprising mixing one ormore compounds of the formula I with at least one further mesogeniccompound and optionally additives.
 16. A compound of formulae I1 to I5:

in which X denotes F, a fluorinated alkyl radical having 1-6 C atoms, afluorinated alkoxy radical having 1-6 C atoms, a fluorinated alkenylradical having 2-6 C atoms, a fluorinated alkylalkoxy radical having 1-6C atoms, L¹⁻⁴ each, independently of one another, denote H or F, andalkenyl denotes an alkenyl radical having 2-6 C atoms; provided that theone or more of the following conditions are met: L¹⁻⁴ have one of thefollowing meanings L¹=L²=L³=L⁴=F, L¹=L²=L³=L⁴=F and L⁴=H, or L¹=L³=F andL²=L⁴=H, and alkenyl is a straight-chain alkenyl having 2 to 5 carbonatoms.
 17. The liquid-crystalline mixture according to claim 1, furthercomprising at least one compound of formula D1, D2 or D3

in which R⁰ denotes an alkyl or alkoxy radical having 1 to 15 C atoms,where, in addition, one or more CH₂ groups in these radicals may each bereplaced, independently of one another, by —C≡C—, —CF₂O—, —CH═CH—,

—O—, —CO—O— or —O—CO— in such a way that O atoms are not linked directlyto one another, and in which, in addition, one or more H atoms may bereplaced by halogen, X⁰ denotes F, Cl, a mono- or polyfluorinated alkylor alkoxy radical having 1 to 6 C atoms, a mono- or polyfluorinatedalkenyl or alkenyloxy radical having 2 to 6 C atoms and Y¹ and Y² eachindependently are H or F.
 18. The liquid-crystalline mixture accordingto claim 1, further comprising at least one compound of formula XIX toXXVII

in which R⁰ denotes an alkyl or alkoxy radical having 1 to 15 C atoms,where, in addition, one or more CH₂ groups in these radicals may each bereplaced, independently of one another, by —C≡C—, —CF₂O—, —CH═CH—,

—O—, —CO—O— or —O—CO— in such a way that O atoms are not linked directlyto one another, and in which, in addition, one or more H atoms may bereplaced by halogen, X⁰ denotes F, Cl, a mono- or polyfluorinated alkylor alkoxy radical having 1 to 6 C atoms, a mono- or polyfluorinatedalkenyl or alkenyloxy radical having 2 to 6 C atoms and Y¹ and Y² eachindependently are H or F.
 19. The liquid crystalline medium according toclaim 1, wherein X is F or OCF₃.
 20. The liquid crystalline mediumaccording to claim 1, wherein X is F.
 21. The liquid crystalline mediumaccording to claim 1, wherein R is H₂C═CHC₂H₄, CH₂CH═CH or CH₂—CH.